Catalyst design for selective-catalytic-reduction (scr) filters

ABSTRACT

Provided is an improved selective catalytic reduction filtering (SCRF) device that separates reduction of nitrogen oxides (NOx) from oxidation of soot, hydro-carbon (HC) and carbon monoxide (CO). The SCRF device has a diesel oxidation catalyst unit for oxidizing HC and CO and oxidizing diesel fuel to support DPF regenerations, and a SCR filtering unit, including at least one inlet channel and being connected to the diesel oxidation catalyst unit, for controlling (soot) emission, cleaning-up slipped HC and CO during a DPF regeneration, and reducing nitrogen oxides in the diesel exhaust gas. At least one inlet channel is coated with an ammonia-neutral oxidation catalyst, and at least one outlet channel is coated with a selected catalytic reduction catalyst.

FIELD OF THE PRESENT TECHNOLOGY

The present technology relates generally to diesel exhaust emissionscontrol. More specifically, the present technology related tosimultaneously controlling nitrogen oxides (NOx), particulate matter(PM), carbon monoxide (CO) and hydrocarbons (HC) utilizing aninnovatively designed SCR filter (SCRF).

BACKGROUND OF THE PRESENT TECHNOLOGY

In a diesel engine, the exhaust gas must be treated properly to removeharmful pollutants before being released to the atmosphere. The exhaustgas passes through a catalytic converter system that includes a DOC(diesel oxidation catalyst), a SCR filter (SCRF), and a selectivecatalytic reduction (SCR) catalyst. The DOC oxidizes carbon monoxide(CO) and hydrocarbons (HC), and nitric oxide (NO) to nitrogen dioxide(NO2). The DOC also behaves like a “diesel burner” to oxidize theinjected diesel fuel to generate exotherm for supporting periodic sootoxidations or diesel particulate filter (DPF) regenerations. The SCRF isa combination of SCR and DPF technologies.

A diesel exhaust fluid (DEF) injection system injects urea solution intothe exhaust for providing ammonia (NH3) to reduce nitrogen oxides (NOx)to harmless nitrogen and water in the presence of the SCR catalyst.Diesel exhaust contains relatively high levels of particulate matters(PM), which is also known as soot. The catalytic converter generallycannot remove elemental carbon, such as soot; soot is usually cleaned upby the DPF. The DPF needs to be regenerated by burning off the sootcollected inside the DPF at temperatures greater than 500° C. The SCR isan individual catalytic converter that reduces the residual nitrogenoxides (NOx) by ammonia (NH3) from the exhaust gas.

The catalytic converter system containing the SCRF, though removingharmful emissions components, is not optimally efficient because of thelow DPF regeneration efficiency, poor CO and HC dean-up activities ofthe SCRF during a DPF regeneration process, and potential contaminationof the SCR catalyst by ash poisoning, HC coking, soot deposition, etc.The present technology is directed primarily to an improved exhaust gastreatment system through an innovative catalyst design of the SCRF.

SUMMARY OF EMBODIMENTS OF THE TECHNOLOGY

The present technology includes a highly efficient diesel exhaust gastreatment system containing a SCRF that enhances DPF regenerationefficiency, NOx reduction reliability, and CO and HC clean-up activitiesduring DPF regenerations.

In one embodiment, the present technology includes a diesel exhaust gastreatment apparatus comprising a diesel oxidation catalyst unit forreceiving diesel exhaust gas, and a selective catalyst reductionfiltering (SCRF) unit, with at least one inlet channel and connected tothe diesel oxidation catalyst unit, for oxidizing CO and HC, reducingnitrogen oxides (NOx), and control PM emission in the diesel exhaustgas, wherein at least one inlet channel is coated with an ammonia (NH3)neutral oxidation catalyst, and at least one outlet channel coated withan selective catalytic reduction (SCR) catalyst using ammonia (NH3) forNOx reduction

In another embodiment, the present technology includes a selectivecatalytic reduction filter (SCRF) comprising at least one inlet channelfor receiving diesel exhaust gas, and at least one outlet channelconnected to the plurality of inlet channels, wherein the at least oneinlet channel is coated with an ammonia-neutral oxidation catalyst, andat least one outlet channel is coated with an selective catalyticreduction (SCR) catalysts.

In yet another embodiment, the present technology includes a method forimproving reduction of nitrogen oxides (NOx) from diesel exhaust gas,comprising receiving the diesel exhaust gas, feeding the diesel exhaustgas with ammonia (NH3), or DEF (urea) solution as an ammonia source,into a selective catalytic reduction filtering device (SCRF) withmultiple inlet channel coated with an ammonia-neutral oxidation catalystand all corresponding outlet channels coated with a selective catalyticreduction catalyst, passing ammonia through the inlet channels to theoutlet channels, and reducing nitrogen oxides (NOx), in the outletchannels of the SCRF.

In an additional embodiment, the present technology includes also amethod for improving the DPF regeneration efficiency with anammonia-neutral oxidation catalyst coated onto all inlet channels wheresoot (or PM) is continuously deposited and accumulated with time beforecarrying out a DPF regeneration. Moreover, with the given oxidationcatalyst, residual CO and HC emissions can be oxidized to effectivelyreduce tailpipe CO and HC emissions during DPF regeneration.

Further features and advantages of the technology, as well as thestructure and operation of various embodiments of the technology, aredescribed in detail below with reference to the accompanying drawings.It is noted that the technology is not limited to the specificembodiments described herein. Such embodiments are presented herein forillustrative purposes only. Additional embodiments will be apparent topersons skilled in the relevant art(s) based on the teachings containedherein.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated herein and form partof the specification, illustrate the present technology and, togetherwith the description, further serve to explain the principles of thetechnology and to enable a person skilled in the relevant art(s) to makeand use the technology.

FIG. 1 is an illustration of a catalytic converter;

FIG. 2 is a schematic of a prior art SCR filter design;

FIG. 3 is a schematic of a SCR filter of the present technology; and

FIG. 4 is a chart illustrating performance of ammonia (NH3) neutralcatalysts, which can be used in a SCR filter of the technology.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE TECHNOLOGY

While the present technology is described herein with illustrativeembodiments for particular applications, it should be understood thatthe technology is not limited thereto. Those skilled in the art withaccess to the teachings provided herein will recognize additionalmodifications, applications, and embodiments within the scope thereofand additional fields in which the technology would be of significantutility. Features described in different embodiments described in thepresent specification may be combined.

FIG. 1 is an illustration of an after treatment system 100 for treatingexhaust gas in a diesel engine. The given system, or a similar system,could be used for treating a lean-burn gasoline engine as well. Theexhaust gas 102 is fed into or enters a diesel oxidation catalyst (DOC)110, passes through a pipe to reach a selective catalytic reduction(SCR) filter (SCRF) 106. The SCRF is a combination of SCR and DPFtechnologies and controls both NOx and particulate matter (PM, alsoknown as soot) emissions simultaneously. After being filtered by SCRF,the exhaust gas continues to the SCR catalyst 104, where more NOx areremoved through reduction.

In the system 100, the SCR catalyst 104 reduces NOx using ammonia (NH3).The ammonia is introduced in the system 100 through a urea solution,e.g. a diesel exhaust fluid (DEF) solution, injected into the exhaustgas stream through a DEF injector 108. The urea solution mixes with thehot exhaust gas and produces ammonia (NH3), which is an agent forreducing NOx in the exhaust gas. Both the inlet and outlet channels ofthe SCRF 106 are coated with SCR catalyst; however, the DPF substrate ofthe SCRF 106 cannot be coated with excess of the SCR catalyst becausethe excessive coating will build up back pressure for the diesel engine.So generally, it is preferred to have a downstream SCR converter 104installed. The downstream SCR converter will further reduce the residualNOx with ammonia.

During a DPF regeneration, DOC is used to oxidize the injected dieselfuel to generate exotherm (heat) for burning off soot accumulated in theinlet channels of a SCRF 106. However, some residual HC and CO will slipout of the DOC 110 when oxidizing the injected diesel fuel. As theexhaust gas mixed with HC and CO slips out from the DOC 110 and the NH3produced from the urea solution moves through the exhaust system andgoes into the SCRF 106, the SCRF 106 removes NOx from the exhaust gas.However, SCRF 106 does not effectively remove HC and CO because the SCRF106 is not equipped with a catalyst agent to effectively oxidize HC andCO. For this purpose, a clean-up catalyst (not shown in FIG. 1) isneeded at the tailpipe to remove residual HC and CO.

When the exhaust gas is injected with the urea solution through the DPFinjector 108, ammonia produced from the urea solution will reduce NOx inthe SCRF 106. As NOx is reduced, soot is also produced and accumulatesin the inlet channels of SCRF 106. So, periodically DPF regeneration isconducted, during which diesel fuel is injected into the exhaust system100 and the DOC 110 will combust the injected diesel fuel to burn offthe soot. A typical SCR catalyst, however, is not effective foroxidizing soot during a DPF regeneration, and this results in anincomplete DPF regeneration. The inefficient DPF regeneration would inturn increase fuel consumption and increase the likelihood foruncontrolled DPF regenerations.

FIG. 2 is a schematic illustration of a conventional SCRF 200 forprocessing diesel exhaust. Exhaust gas 202 flows into the inlet channels210 of the SCRF 200. The inlet channels 210 are coated with a SCRcatalyst 206, such as a Zeolite-based SCR catalyst. The exhaust gas 202is mixed with ammonia produced from the urea solution for reducing NOxin SCRF; in the meantime, the exhaust soot is deposited in the inletchannels of the SCRF 200 and the soot-filtered exhaust gas 202 exitsthrough the outlet channel 208. The SCRF 200 with urea cansimultaneously control NOx and PM emissions in diesel exhaust; however,a typical urea SCR catalyst, such as a zeolite-based SCR, is inactivefor hydrogen-carbon (HC) and carbon monoxide (CO) oxidation at lowexhaust temperature (<350 C). Moreover, a SCR catalyst is relativelyinactive for catalyzing soot oxidation during a DPF regeneration, whichresults in low DPF regeneration efficiency.

Because of soot buildup inside the SCRF 200, the efficiency of NOxremoval through a urea SCR catalyst coated on the inlet channels 210 maybe reduced and the durability of SCR catalyst can be affected bycontaminations from HC coking and ash depositions. The soot buildup alsorequires DPF regeneration to be conducted periodically. During the DPFregeneration, diesel fuel is injected into the exhaust gas and the sootis burned off. If the soot accumulation occurs at a higher rate, morefrequent DPF regeneration would be needed and consequently, there is agreater likelihood of the uncontrolled DPF regeneration being affectedby driving condition.

The shortcomings of the SCRF of FIG. 2 are overcome by an improved SCRFcatalyst design according to the present technology. FIG. 3 is aschematic illustration of an improved SCRF300. The inlet channels 310 ofthis SCRF are coated with an ammonia-neutral oxidation catalyst (ANOC)306. The ANOC 306 is basically inactive or non-selective for ammoniaoxidation when temperature is below 400 C but still maintains HC and COoxidation capability as a special type of DOC catalyst for temperaturebelow 400 C. By coating the inlet channels 310 with an ANOC catalyst 306washcoat, the ammonia produced via DEF (urea solution) injection cansafely penetrate through the inlet channels 310 without being oxidizedwhen temperature is lower than 400 C. The outlet channels 312 of the newSCRF are coated by a washcoat of urea SCR catalyst, such as azeolilte-based SCR.

As illustrated in FIG. 3, the exhaust gas 302 with ammonia flows intothe inlet channels 304 of the SCRF 300 and the ammonia can pass throughthe channels coated with ANOC 306 and reach the outlet channels 308. Inthe outlet channels 308, the ammonia reduces nitrogen oxides (NOx) ofthe exhaust gas in the presence of SCR catalyst 106 before the exhaustgas is released. The SCR catalyst is not consumed during reduction ofNOx. As the exhaust gas flows into the inlet, coated with the ANOC 306,HC and CO oxidation occurs, as well as soot deposition, HC coking, andash accumulations. The NOx in the exhaust gas passes through the inletchannels 310 substantially unaffected and reaches the outlet channels312 that are coated by the SCR catalyst 106. The SCR NOx reductionactivity occurs more reliably in the outlet channels because the NOxreduction occurs separately from the soot deposition, HC coking, and ashaccumulation.

Because the reduction of NOx occurs in the outlet channels 312 when theexhaust gas passes through the outlet channels and the soot deposition,HC coking, and ash accumulation mostly occur in the inlet channels 310,the SCR washcoat, in the outlet channels 312, is prevented from beingcontaminated by ash and masked by HC coking. As a result, thereliability and durability of the SCRF 300 is enhanced.

In the improved SCRF catalyst design, the DPF regeneration efficiency isenhanced by an ammonia-neutral oxidation catalyst (ANOC) along with moreeffective HC and CO slip control. As a result, lower vehicle fuelconsumption can be realized and DPF regeneration need to be conductedless frequently, thus reducing the possibility of DPF regeneration beingaffected by vehicle driving conditions, such as an uncontrolled DPFregeneration at an idling condition.

FIG. 4 is a chart 400 illustrating performance of ammonia-neutralcatalysts related to the present technology. The X-axis indicates inlettemperature in degrees centigrade (C) of ammonia-neutral catalystsincluding a reference DOC catalyst. The left Y-axis indicates an ammonia(NH3) conversion percentage and the right Y-axis indicates a percentageby which nitrogen oxides (NOx) are remade via ammonia oxidation.

Bars 402 represent performance of a conventional DOC at differenttemperatures. Bars 404 represent performance of an improvedammonia-neutral oxidation catalyst (ANOC) at different temperatures. Ascan be seen, NOx remaking does not start until the temperatureapproaches about 400° C. for the given ANOC. In contrast, ammonia (NH3)conversion and NOx remaking start at a much lower temperature ofapproximately 250° C. for the typical DOC.

It is to be appreciated that the Detailed Description section, and notthe Summary and Abstract sections, is intended to be used to interpretthe claims. The Summary and Abstract sections may set forth one or morebut not all exemplary embodiments of the present technology ascontemplated by the inventor(s), and thus, are not intended to limit thepresent technology and the appended claims in any way. It is within thescope of the technology that different embodiments described in thepresent specification may be combined.

What is claimed is:
 1. An exhaust gas treatment apparatus comprising: adiesel oxidation catalyst unit for receiving exhaust gas; and aselective catalyst reduction filtering unit, having at least one inletchannel and being connected to the diesel oxidation catalyst unit, foroxidizing hydrocarbon and carbon monoxide, reducing nitrogen oxide, andcontrolling particulate matters emission in the exhaust gas, wherein theat least one inlet channel is coated with an ammonia-neutral oxidationcatalyst.
 2. The exhaust gas treatment apparatus of claim 1, furthercomprising at least one outlet channel coated with a selective catalyticreduction catalyst.
 3. The exhaust gas treatment apparatus of claim 2,wherein the selective catalytic reduction catalyst includes azeolilte-based catalyst.
 4. The exhaust gas treatment apparatus of claim1, further comprising a selective catalytic reduction catalyst connectedto the selective catalyst reduction filtering unit, for reducing thenitrogen oxides.
 5. The exhaust gas treatment apparatus of claim 4,wherein the selective catalyst reduction unit is coated with azeolilte-based catalyst.
 6. The exhaust gas treatment apparatus of claim1, further comprising a diesel exhaust fluid injector for injecting aurea solution into the exhaust gas.
 7. The exhaust gas treatmentapparatus of claim 6, wherein the diesel exhaust fluid injector islocated between the diesel oxidation catalyst unit and the selectivecatalyst reduction filtering unit.
 8. A selective catalytic reductionfilter comprising: at least one inlet channel for receiving exhaust gas;and at least one outlet channel connected to the at least one inletchannel, wherein the at least one inlet channel is coated with anammonia-neutral oxidation catalyst.
 9. The selective catalytic reductionfilter of claim 8, wherein the at least one outlet channel is coatedwith a selective catalyst reduction catalyst.
 10. The selectivecatalytic reduction filter of claim 9, wherein the selective catalyticreduction filter is coated with a zeolilte-based catalyst.
 11. Theselective catalytic reduction filter of claim 8, wherein, when theselective catalytic reduction filter is in operation, nitrogen oxides inthe exhaust gas pass through the at least one inlet channelsubstantially unaffected by the ammonia-neutral oxidation catalyst. 12.The selective catalytic reduction filter of claim 9, wherein, when theselective catalytic reduction filter is in operation, the exhaust gas ismixed with ammonia and nitrogen oxides in the exhaust gas is reducedwhen nitrogen oxides react with ammonia when the exhaust gas passesthrough the at least one outlet channel coated with the SCR catalyst.13. The selective catalytic reduction filter of claim 8, wherein theexhaust gas is from a diesel engine.
 14. The selective catalyticreduction filter of claim 8, wherein the exhaust gas is from a lean-burngasoline engine.
 15. A method for reducing nitrogen oxides from anexhaust gas, comprising: feeding an exhaust gas with ammonia into aselective catalytic reduction filtering device comprising at least oneinlet channel coated with an ammonia-neutral oxidation catalyst and atleast one outlet channel coated with a selective catalytic reductioncatalyst; passing the ammonia through the at least one inlet channel tothe at least one outlet channel; and reducing, in the at least oneoutlet channel, nitrogen oxides in the exhaust gas.
 16. The method ofclaim 15, further comprising injecting a urea solution into the exhaustgas to produce the ammonia.
 17. The method of claim 15, furthercomprising oxidizing, in the at least one inlet channel, hydro-carbonand carbon-monoxide in the exhaust gas.
 18. The method of claim 15,wherein reducing the nitrogen oxides occurs at temperatures below about400C.
 19. The method of claim 15, wherein the selective catalyticreduction catalyst comprises a zeolilte-based catalyst.
 20. The methodof claim 15, further comprising receiving the exhaust gas a dieseloxidation catalyst.